Electric and hybrid electric vehicles employ a high voltage (HV) energy storage device (ESD), such as a high voltage traction battery, and a power conversion system to provide an alternating current (AC) output to a motor. With an adequately high voltage, the HV ESD can provide power to assist in motoring operations of the vehicle, thereby reducing its dependence on a fossil-fueled internal combustion engine. Because it is designed to provide motoring power, an HV ESD can have a considerably higher voltage than a standard auxiliary vehicle battery used to power low voltage vehicle systems. As a result, an HV ESD and its positive and negative rails are typically isolated from the vehicle's low voltage systems. Furthermore, as a precautionary measure, an HV ESD, unlike the auxiliary battery, is usually not grounded to a vehicle chassis, but is instead typically configured in an isolated return circuit. Despite best efforts to isolate the HV ESD and the HV AC system, on occasion, electric current may flow through an unintended path. For example, a short circuit or low impedance connection, i.e. a ground fault, may occur between an AC motor current and a chassis. If so, vehicle equipment coupled to the high voltage buses may experience extreme swings in voltage and current, and may even be significantly damaged.
In the past, various methods and systems have been proposed to detect faults in a vehicle's electrical system. For example, U.S. Pat. No. 5,382,946 issued to Gale (hereinafter “Gale”), teaches a method and a circuit for measuring the leakage path resistance in an electric vehicle having an isolated high voltage traction battery and an auxiliary battery grounded to the vehicle. The circuit operates by periodically applying a selected excitation signal and comparing the voltage induced on an energy storage element by the excitation signal to a selected reference voltage during a selected time period. An alternative embodiment provides a circuit that operates by applying a periodic excitation signal and comparing the phase shift of the voltage induced on an energy storage element to the phase shift of a signal derived from the excitation signal. While fit for their intended purposes, the solutions suggested by Gale are directed primarily towards detecting leakage between a traction battery and an auxiliary battery grounded to the vehicle. Leakage from a vehicle's AC system, such as leakage at the output of an inverter to a chassis is not addressed.
In contrast to Gale, other proposals have considered AC current leakage. For example, U.S. Pat. No. 6,856,137, issued to Roden et al. (hereinafter “Roden”), suggests an AC ground fault detector system that senses an AC signal indicative of an unintended electrical path between a load driven by a power source and a reference potential using a capacitively coupled circuit. A first power conductor is coupled to a first terminal of the power source and a second power conductor is coupled to a second terminal of the power source. A switching mechanism coupled to the first and second power conductors is operative for alternately connecting a phase of the load with the first and second power conductors according to a predetermined switching rate, whereby, during normal operation, voltages developed at the first power conductor and second power conductor are substantially constant with respect to a reference potential. In the event of an occurrence of the unintended electrical path of at least one phase of the load with the reference potential, time varying voltages are developed at the first power conductor and second power conductor associated with the switching rate. A fault detector senses presence of a square wave voltage caused by the fault through a series capacitive/resistive circuit. Roden also teaches that high voltage isolation can be performed through a sense capacitor or transformer while sensing the voltage change indicative of the ground fault condition. Roden thus teaches detection of a time-varying voltage on a DC bus to detect a ground fault.
U.S. Pat. No. 7,443,643 issued to Kubo (hereinafter “Kubo”), teaches an inverter device that comprises a ground fault detection circuit connected between a negative line of the battery and the vehicle body, and a controller, wherein the ground fault detection circuit includes a serial circuit of a resistor element and a condenser element or a serial circuit of a plurality of resistor elements, and a potential at a connection point of the elements is input to the controller to detect a ground fault. The Kubo ground fault detection circuit is connected between the negative line of the battery and the vehicle body. The Kubo inverter device judges that a ground fault has occurred in the negative line of the battery when the input potential has decreased, and judges that ground fault has occurred in a positive line of the battery or in an output of the inverter when the input potential has increased.
As a final example, U.S. Pat. No. 8,022,710 issued to Ivan et al. (hereinafter “Ivan”) teaches methods for AC fault detection. In general, Ivan teaches a method for detecting an AC fault caused by a module coupled to a bus of a hybrid/electric power train system. When a high voltage DC input signal is received from the bus, the differential mode voltage component is removed from the high-voltage DC input signal to generate a common mode AC voltage signal. A magnitude of the common mode AC voltage signal is measured and it is determined whether the measured magnitude of the common mode AC voltage signal is greater than or equal to a fault detection threshold voltage. Thus Ivan teaches detection and removal of a differential mode voltage from a DC input signal.